Zirconia single crystals, being glossy and having transparency, are used in jewelry goods, etc. as cubic zirconia. However, single crystals suffer from difficulty in processing into desired shapes and have problems, such as limited size that can be obtained.
Under such circumstances, attempts have been made to develop polycrystalline bodies having transparency comparable to single crystals by molding powders into desired shapes, followed by sintering.
For example, Patent Document 1 reports a zirconia sintered body produced by HIP treatment and having high transparency substantially comparable to single crystals. However, the sintered body disclosed has high translucency but is not suitable for use in applications where strength is particularly required because of its large grain size of about 50 μm.
Also, Patent Document 2 reports a highly transparent zirconia sintered body containing titania. However, the added titania promotes the growth of the grains to the grain size as large as about 100 μm to about 200 μm, which causes insufficient mechanical strength of the sintered body disclosed.
That is, since the conventional transparent zirconia sintered bodies have large grain sizes of 50 μm or more, their mechanical strengths are low. For example, a transparent titania-containing zirconia sintered body having a grain size of 200 μm has a flexural strength as low as 210 MPa (Non-Patent Document 1).
High-strength zirconia sintered bodies have insufficient translucency. For example, a zirconia sintered body having an average grain size of 10 μm or less has an in-line transmission of less than 20% in the case of a sample thickness of 0.5 mm (Patent Document 3), a zirconia sintered body has a total forward transmission of less than 50% in the case of a sample thickness of 0.5 mm (Patent Document 4), and a zirconia sintered body has a total forward transmission of less than 50% in the case of a sample thickness of 1 mm (Patent Document 5).
As just described, transparent zirconia sintered bodies which satisfy both high translucency and high mechanical strength have not yet been obtained.